Electric blasting cap



y 30, 1944- R. w. LAWRENCE -2,350,172

ELECTRIC BLASTING CAP F iled May 1'7. 1940 FIG-2 ROBERT W. LAWRENCE INVENTOR MA Q). Min-4&8,

ATTORNEY Patented May 30, 1944 2,s o,172 ELECTRIC BLASTING car Robert W. Lawrence, Wilmington, Del., asslgnor to Hercules Powder Company, Wilmington, Del., a corporation of Delaware Application May 17, 1940, Serial No. 335,706

4 Claims.

-This invention relates to an improved electric blasting cap and more particularly to a blasting cap which is highly resistant to strong external heating. The cap of this invention is so designed as to prevent detonation when the cap is exposed to strong external heat.

As is well known, electric blasting caps consist of a casing in which may be confined at the bottom a detonating charge, for example, tetryl, trinitrotoluene, nitrostarch, or the like, and has superimposed upon this base a priming charge, for example, diazodinitrophenol, mercury fulminate, lead azide, or mixtures thereof with oxidizing agents such.as potassium chlorate. This priming charge is normally fired by a bridge wire assembly which may be either inserted directly into the priming charge or may be imbedded in an ignition charge, which will in turn fire the priming charge. The cap may also be fired by a cavity type plug, for example, a plug containing an easily ignited material that fires into the explosive compound or by a match head assembly. The cap shell is closed by a waterproof composition such as, for example, asphalt above the firing assembly and then is further closed by a sealing compound, for example, sulfur.

The electric blasting caps now known to the art,are very sensitive to external heating and it is known that these caps may be accidentally exposed to strong external heating. Thus, it will be apparent that during coal mining operations a misflred cap might remain in the coal and in time find its way into a stove or furnace. The blasting caps of present design would usually detonate under these conditions and cause extensive damage with possible loss of life. Throughout industry, there are other places at which caps may be exposed to strong external heating and which may be the direct cause of serious damage. For example, if misfired caps from a rock quarry are introduced into a calcining furnace or if caps from It is an object of this invention to produce an v electric firing device which may be heated externally to a high temperature without causing detonation.

A further object of this invention is to manu facture a firing device that is less sensitive to heat than known devices.

A further object of this invention is to provide an explosive charge for use in firingdevices which will tend to decompose and not detonate upon application of strong external heating.

Further objects will be apparent hereinafter.

In accordance with this invention, the objects have been accomplished by manufacturing electric blasting caps in which there is a normal base charge, for example, tetryl, trinitrotoluene, pentaerythritoltetranitrate, nitrostarch, or the like, and which have a priming charge consisting of an admixture of lead azide and a suitable secondary explosive, which is decomposed by heat, such, for example, as solid nitrated polyhydric alcohols, solid nitrated sugars, solid nitrated aromatics, hexogen, and their equivalents. It has been found that electric blasting caps having the proper admixture of lead azide and one of the above mentioned types of secondary explosives as a priming charge may be exposed to strong external heating without the detonation of the cap. In order to cause detonation of the caps of this invention, it has been found that it is necessary to use either a cavity type plug containing any suitable strong ignition compound'as the ignition assembly or an ignition assembly of the match head type.

It is believed that the resistance to detonation upon heating of the caps of this invention results because the secondary explosive in the priming admixture is melted or decomposed by the external heat and thus exerts a desensitizing eflect on the lead azide which is the priming explosive. Further, to support this theory, but not to limit the invention, it may be noted that the ignition temperature of lead azide is in the neighborhood of 300330 C. This may be contrasted with the much lower ignition temperatures of initiating compounds such as mercury fulminate and dialw dinitrophenol which range from I'm-180 C. Most secondary explosives decompose fairly rapidly at temperatures of 200 C. or moderately higher. Inv general, the fusion of these compounds occurs at a somewhat lower temperature.

The secondary explosives which I have found will resist detonation when in admixture with lead azide and used as a priming explosive, may be those decomposable compounds such as, for example, solid nitrated polyhydric alcohols such as pentaerythritoltetranitrate, and nitromannite; solid nitrate'd sugars such as nitrolactose, nitrosucrose, and nitromaltose; and solid nitrated aromatics such as tetryl (tetranitromethylaniline). T. N. T. (trinitrotoluene), and pentryl (trinitrohenylnit'ramino an nitrate); and hexogen riming charge composed of an admixture of lead zide and a secondary explosive whose ignition ssembly comprises a match head type firing deice.

The caps shown in Fig. 1 include a copper shell r casing I, a base charge 2 positioned in the base f the shell I, a priming charge 3 superimposed .pon the base charge 2, an ignition assembly l omposed of an ignition charge 4, and a'high esistant bridge wire-5 in a cavity type plug 6. .he cap also has within the shell I a waterproofrig material I, and a sulfur seal 8. From the ridge wire 5 extend two leg wires 9 through rhich the firing current is conducted.

In the cap of Fig. 1, the base charge 2 may be omposed of any high explosive which is not eadily brought to detonation by strong external ieating, such, for example, as tetryl, trinitrooluene, pentaerythritoltetranitrate, nitrostarch ind the like, and the priming charge 3 may be omposed of an admixture of lead azide and a uitable secondary explosive such as pentaerythitoltetranltrate, nltrolactose. tetryl or other solid econdary explosives which are decomposed b ieat.

In Fig. 2 the explosive charges are similar to hose depicted in Fig. 1. The remaining contruction of Fig. 2 comprising the ignition assemly H is similar to the ignition assembly III of lg. 1, but is of the match head type.

In order to further explain the manufacture .nd properties of the caps of this invention, sevral specific embodiments and the tests conducted hereon are described in detail below.

In testing caps of my invention in comparison with commercial caps, the following procedure was followed: Individual caps were \placed in a ath of molten metal that was maintained at a lefinite temperature by suitable means such as wan electrical hot plate and a thermocouple.

.his molten metal bath was so constructed that he cap could be introduced quickly therein to deermine its resistance to strong external heating. The manufacture of a specific cap in accord- .nce with this invention was carried out as folows: A .30 inch caliber 1%" bronze shell was oaded with an explosive charge which consisted f a base charge of .30 gram of tetryl pressed into ilace with 4000 lbs./sq. in. pressure and a primng charge of 0.40 gram of an admixture of ead azide and pentaerythritoltetranitrate 70/30 rressed at 2000 lbs/sq. in. pressure. The ignition .ssembly comprised a cavity plug containing a lash powder consisting of mercury fulminate i9.1%, potassium chlorate 47.1%, nitrostarch 4.1%. and charcoal 9.4%. A waterproof-comnosition and a sulfur seal completed the cap.

The results obtained with the cap described .bove are listed in Table 1 as can 1. There are .lso listed in Table 1 the compositions of various -ther caps made in accordance with this invenlon with the results obtained when they werentroduced into the molten metal bath.

Priming cherge=0.27 gm. tetryl/lead Table I Results of test c n 1 Base charge=0.30 gm. tetr yl Priming charge= 0.40 gm. P. E. T.

NJIead atide /70 'Ignitlon by cavity type plug Base charge=0.3l gm. tetryl Pr im l'ng charge ozi gm nitrolactose/lead ends 30/70 Ignition by cavity type plug Cap 8 A Base charge 0.30 gm. tetryl Priming charge-=0J5 gm nitromannite/lead azide 40/60 Ignition by cavity type plug 1 Base charge=0.3l gm. tetryl azide 40 Ignition by cavity type plug Cap 6 Base charge=0.30 gm. tetryl Priming chsrge==0.36 gm. '1. N. '1./lesd azide 40/00 Ignition by cavity type plug Cap 7 Base charge =01!) gm. mm Priming charge-0.21 gm. bexogen/lesd snide Ignition by cavity type plug,

Five caps tested decomposed in 61. 71, 75, 76, and 80 seconds respectlvely.

Five caps tested decomposed in 61, 66, 70, 71, and 72 seconds.

Five caps tested decomposed in 60, 52, 63, 54 and 55 seconds.

Five caps tested decomposed in 71, 72, 80, 90 end 98 seconds.

Five caps tested decomposed ln 61, 03, 64, and 66 seconds.

Five caps decomposed in .72, 72 and 73 seconds.

Five caps tested decomposed in 69, 72, 73, 74 and 78 seconds.

For purposes of comparing the caps of this invention with various types of commercial caps, the following Table 21s given:

Table 2 7 Composition of cap tested Results of test Cap A Base charge =0.35 gm. tetryl/diazo 25 Five caps tested shot in Priming charge-0.45 gm. di in 6, l5, 16, 19 and '20 trophenol/ch crate 75/25 seconds.

Ignition by bridge wire Cap B 13? charge-0.33 gm. humiliate/chlorate Five caps tested shot 28,

20 Primixicharge =0.67 gm. lulminate/chlorate Ignition by bridge wire Cap 6' Base charge-0.30 gm. tetryl Five caps tested shot 51, Priming charge-0.21 gm. lead azide 57, 58, 62, and 70 Ignition by cavity type plug 1 seoonds.

' Cap D Base charge=0.30 gm. nitrosterch Five caps tested shot 11, Pnmingcherge==0.92 gm. fulminate/chlo- 17, 26, 28, and 30 I rate 80/2) seconds. Ignition by bridge wire ap E Base charge-0.23 gm. mm .Five caps tested shot 16, Pnminggjcharge-OtigmJulminate/ehlm 2;, 27, 28 and 29 rate co d Ignition by match head Se n s The above Tables 1 and 2 conclusively show that the use of an explosive charge comprised of a base charge, a lead amide-secondary explosive priming charge and an'indirect ignition assembly produce a cap which will not detonate upon 30,31,34and 39seeonds.

' Lead aside/P. a. 'r. N. 50 50 strong external heating while the commercial caps will.

In the cap indicated as cap 1, Table 1, the proportions of secondary explosive and lead azide used was 30/70, but it has been found that it is possible to obtain a cap which is resistant to strong external heating by using a mixture of these two compounds which contain the secondary explosive within the range of about 30% to about 60%. It will be noted from the above tables that all of the commercial caps detonated when placed in the molten metal bath, while the caps prepared inaccordance with this invention decomposed quietly when introduced into the bath.

The range of about 30% to about 60% secondary explosive determines to a large degree the resistance of the caps to strong external heating. This is shown in thefollowing table which lists the resistance of various admixtures of lead azide and pentaerythritoltetranitrate to external heating. While the proportions affect the resistance to heat, the initiating characteristic of the mixture is the factor which determines the most preferable mixture to use as the priming charge.

Table 3 Priming charge composition Result Shot after 51, 57, 58, 62 and 70 seconds.

Shot after 52, 63, 69 and 71 seconds. Decomposed after 64 seconds.

Shot after 62, 76, 76, 77 and 81 seconds.

Decomposed after 66, 71, 75,

76 and 80 seconds.

Decomposed after 74, 77, 78,

78 and 81 seconds.

Decomposed after 70, 74, 87,

88 and 89 seconds.

Decomposed after 75, 77, 77,

83 and 86 seconds.

Lead nzide/P. E. T. N. 80/20 Leadazlde/P. E. T. N. 70/30 Lead azidcfP. E. T. N. (30/40 Lead azlde/P. E. T. N. 40/60 From the above Table 3 it is clearly shown that I may use a priming charge comprising an admixture of a secondary explosive between about 30% and about 60% by weight and lead azide between about 40% and about 70% by weight, and manufacture an electric blasting cap which is resistant to strong external heating.

In the case of the P. E. T. N.-lead azide mixture discussed above, the use of more than 60% P. E. T. N. would result in a priming mixture which would be hard to ignite and would have a very high minimum priming charge. With nitrolactose and nitrosucrose the upper limit would extend past 60% since these compounds are more easily ignited than P. E. T. N., so consequently a greater proportion could be used in admixture with the lead azide. When very large percentages of nitromannite are used, the resulting priming charge, though it can be ignited, requires a reenforcing capsule for satisfactory firing results. The upper limit, therefore, depends upon the properties of individual compounds used with the lead azlde, but the percentage used will usually be between 30 and by weight.

I have found that the secondary explosive and lead azide should be of similar screen size to effect proper admixture and prevent segregation. Thus,

if material which will pass a #0 bolting cloth is used, a very good admixture is obtained which will not separate and the caps. Although I have found material passed through a #0 bolting cloth operable, I prefer to use finer material, for

which will charge easily into example, material having a particle size of about 50 microns as this size gives better priming action to the base charge. In effecting the detonation of the explosive charge of the caps of this invention, it is necessary to use a rather strong ignition material. This material may, of course, upon heating deflagrate or detonate, but by the use of a cavity type firing plug, the possibility of the ignition compound detonating is entirely removed. The cavity type firing plug .protects the ignition charge from the strong external heating until the main explosive charge is completely decomposed. Since the ignition charge present within the cavity is very small, being approximately only .02 of a gram, there is absolutely no danger if this small amount of explosive detonates after the main charge has been decomposed. The above discussion concerning the cavity type firing plug also holds true if the means of ignition is a match head.

The advantage of the use of this type blasting cap in the coal industry will at once become apparent. Thus, if caps used in the production of coal accidentally are placed in a stove or furnace, the detonating compound contained therein will only decompose and no damage will result. These caps will also be of use in other industries where the introduction of a blasting cap into equipment will result in damage.

It will be understood that the details and examples hereinbefore set forth are illustrative only, and that the invention as broadly described and claimed is in no way limited thereby.

What I claim and desire to protect by Letters Patent is:

1. An electric firing device resistant to detonation upon application of strong external heating having in combination a closed-end metallic shell; a base charge disposed in the closed end of said shell; a priming charge disposed above the base charge and comprising an intimate admixture of lead azide and a secondary explosive selected from the group consisting of solid explosive nitrated polyhydric alcohols, solid explosive nitrated sugars, solid explosive nitrated aromatics, and hexogen, the said secondary explosive being present in amount from about 30 to about 60% by weight of the said priming charge and adapted to desensitize the priming charge at a temperature below the ignition temperature of the lead azide; and an ignition charge disposed in a cavity type plug and above the priming charge forming an ignition assembly adapted to lower the rate of heat transfer external to said device to said ignition charge; said device adapted to retard ignition of said ignition chargeuntil said priming charge is desensitized, and to render said firing device resistant to detonation, upon application of strong externalheating.

2. An electric firing device resistant to detonation upon application of strong external heating having in combination a closed-end metallic shell; a base charge disposed in the closed end of said shell; a priming charge disposed above the base charge and comprising an intimate admixture of lead azide and pentaerythritoltetranitrate, the said pentaerythritoltetranitrate being present in amount from about 30 to about 60% by weight of the said priming charge and adapted to desensitize the priming charge at a temperature below the ignitior temperature of the lead azide; and an ignition charge disposed in a cavity type plug and above the priming render said firing device resistant to detonation,

upon application ofstrong external heating.

3. An electric firing device resistant to detonation upon application of strong external heating having in combination a closed-end metallic shell; a base charge disposed in the closed end of said shell; a priming charge disposed above the base charge and comprising an intimate admixture of lead azide and tetryl, the said tetryl being present in amount from about 30 to about 60% by weight of the said priming charge and adapted to desensitize the priming charge at a temperature below the ignition temperature of the lead azide; and an ignition charge disposed in a cavity type plug and above the priming' charge forming an ignition assembly adapted to lower the rate of heat transfer external to said device to said ignition charge; said device adapted to retard ignition of said ignition charge until said priming charge is desensitized, and to'renden said firing deviceresistant to detonation, upon application of strong external heating.

4.- An electric firing device resistant to detonation upon application of strong external heating 5 having in combination a closed-end metallic shell; a base charge disposed in the closed end of said shell; a priming charge disposed above the base charge and comprising an intimate admixture of lead azide and nitrolactose, the said nitrolactose being present in amount from about 30 to about 60% by weight of the said priming charge and adapted to desensitize the priming charge at a temperature below the ignition temperatur of the lead azide; and an ignition charge disposed in a cavity type plug and above the priming charge forming an ignition assembly adapted to lower the rate of heat transfer external to said device to said ignition charge; 'said device adapted to retard ignition of said ignition 20 charge-until said priming charge is desensitized, and to render said firing device resistant to detonation, upon application of strong external heating.

ROBERT W. LAWRENCE. 

